Fuel (gasoline) for a gasoline engine includes regular gasoline of which octane value is low and high-octane gasoline. As shown in JP-9-158819A, it is known that a control apparatus determines the octane value of fuel based on knocking intensity of the internal combustion engine and adjusts an ignition timing to a proper timing according to the determined octane value.
An internal combustion engine includes a gasoline engine which combusts gasoline, a diesel engine which combusts light oil, and the like. Recently, since a self serve gas station where a driver serves fuel by himself/herself is increasing, there is a possibility that the driver mistakenly serve light oil, which is improper fuel, to a gasoline engine. Besides, there is a possibility that low grade fuel may be served intentionally, which is a mixture of gasoline and improper fuel such as light oil or kerosene. Since regular gasoline and high-octane gasoline are same type fuel, even if regular gasoline is served to an engine for high-octane gasoline, the engine can be driven according to the technology shown in JP-9-158819A.
Even if improper fuel such as light oil or kerosene, or low grade fuel is served to a gasoline engine, some amount of gasoline remaining in a fuel tank is supplied to the engine with the improper grade fuel. Hence, in an electric engine control system, the engine can be driven by controlling the ignition timing if a mixing rate of the improper fuel is low. As the mixing rate of the improper fuel is increased, the combustion state of the engine is deteriorated, so that the engine rotation becomes unstable and a drivability is deteriorated. In the worst case, the engine may be stopped.
Besides, even if improper fuel or low grade fuel is served into the fuel tank where gasoline scarcely remains, the gasoline remaining in a fuel pipe is supplied to the engine at beginning, so that the engine can be driven properly for a while. After that, when the gasoline remaining in the fuel pipe is totally combusted and the improper fuel or the low grade fuel is injected into the engine, the combustion state of the engine is deteriorated so that the engine may be stopped in the worst case.